Recently, research on software-defined radios is being intensively conducted (see Japanese Patent Nos. 3686736, 3439973, and 3517097, Japanese Unexamined Patent Publication JP-A 11-284409 (1999), and Japanese Patent No. 3420474). For example, it is possible to change a mobile terminal between multiple modes such as car navigation apparatuses and terrestrial television receiving terminals, by replacing existing software of the mobile terminal to change the configuration thereof. Development in increasing the scale of field programmable gate arrays (abbreviated as FPGAs), increasing the speed of digital signal processors (abbreviated as DSPs), putting reconfigurable processors (abbreviated as RCPs) into practice, increasing the speed of A/D (analog to digital) or D/A (digital to analog) converters, and increasing the speed of data transmission interfaces has made a significant contribution to the realization of this technique for software-defined radios (see OKI “Technical Review” October 2005, OTR204, Vol. 72, No. 4, pp. 80-85, and “IEICE Technical Report” ED2005-116, OME2005-42(2005-09), pp. 45-50, for example).
In particular, FPGAs have made a significant contribution to putting software-defined radios into practice, and constitute a core technique thereof. FPGAs can support various modulation and demodulation processes, by changing the processing itself of digitalized signals by means of programmable circuits. Thus, as the premise, the radio portion is required to have a wide band.
However, it is difficult to realize programmable center frequency and passband of wideband antennas or filters that are required for this method. Thus, it is necessary to prepare a filter bank and switch a plurality of filters as needed. Furthermore, a direct conversion method and the like have been also investigated (see OKI “Technical Review” October 2005, OTR204, Vol. 72, No. 4, pp. 80-85, and “IEICE Technical Report” ED2005-116, OME2005-42(2005-09), pp. 45-50).
In conventional techniques, the passband of a radio portion, namely, a high frequency circuit component such as an antenna or a filter is limited, or a plurality of types of such high frequency circuit components are provided and selectively used. With radio portions in which the passband is limited, a software-defined radio cannot be realized that can be changed between multiple modes. With radio portions in which a plurality of types of high frequency circuit components are provided and selectively used, the structure of the high frequency circuit components becomes large and complex, which lacks versatility.
A technique using the direct conversion method has the following problem. The transmission side needs to have a function of converting a digital signal transmitted from a signal processing portion into an analog signal, and upconverting the signal to a desired radio frequency over a wide band. On the reception side, in a case where a plurality of high level signals are inputted to a desired band in a frequency converting portion, there is the problem that the dynamic range is lowered and a non-linear distortion of a mixer occurs.